Tamper detection

ABSTRACT

A vehicle (18) having a license plate tamper detection system comprising: a first sensor (4) arranged to provide monitor data corresponding to presence of a first license plate (20) on the vehicle; and a second sensor (6) arranged to provide reference data. The license plate tamper detection system is arranged to compare the monitor data with the reference data and generate a tampering event if the comparison yields more than a predetermined difference between the monitor data and the reference data.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/245,084, filed Aug. 23, 2016, which claims priority to Great BritainPatent App. No. 1514987.5, filed Aug. 24, 2015, the contents of all ofwhich are fully incorporated by reference herein for all purposes.

BACKGROUND

All around the world, motor vehicles are fitted with license plates(sometimes referred to as registration plates or number plates) toidentify them for official purposes. Each license plate comprises anidentifier (numeric or alphanumeric) unique to the issuing region thatcan be used to permit easy identification of the vehicle and byextension details surrounding the vehicle such as its history and itsowner. In some countries the license plate identifiers are uniquenationally, while in others the identifiers are unique to a particularstate or province.

Unfortunately, due to their uniquely identifying nature, the ability toable to steal or tamper with (i.e. remove or attempt to remove) licenseplates is particularly attractive to criminals. Examples of crimesrelating to license plate tampering include masking the identity ofstolen vehicles, fleeing from fuel stations without paying and evadingparking tickets, speeding fines, congestion charges or tolls.

There have been a number of solutions proposed in the past to try andcounter license plate related crimes. A common approach is to use tamperproof screws (sometimes referred to as “security screws”) to try to makeit difficult to remove license plates from a vehicle. However, these areeasily overcome and therefore of limited effectiveness.

BRIEF DESCRIPTION OF THE FIGURES

Embodiments will now be described, by way of example only, withreference to the accompanying drawings in which:

FIG. 1 is a block diagram of a tamper detection system in accordancewith an embodiment of the present disclosure;

FIG. 2 shows the placement of two sensors on a vehicle within anembodiment incorporating the tamper detection system of FIG. 1;

FIGS. 3A and 3B show a sensor embedded within a license plate within thetamper detection system of FIG. 1;

FIG. 4 shows the placement of two sensors on a vehicle within a furtherembodiment incorporating the tamper detection system of FIG. 1;

FIG. 5 shows the tamper detection system of FIG. 4 communicating with ahandheld enforcement device; and

FIG. 6 shows the tamper detection system of FIG. 4 communicating with anautomatic interrogation system.

FIGS. 7A and 7B show a time-of-flight based sensor mounted behind alicense plate within the tamper detection system of FIG. 1; and

FIG. 8 shows time-of-flight based sensors embedded within two licenseplates within the tamper detection system of FIG. 1.

DETAILED DESCRIPTION

When viewed from a first aspect, a vehicle is provided having a licenseplate tamper detection system comprising:

a first sensor arranged to provide monitor data corresponding topresence of a first license plate on the vehicle; and

a second sensor arranged to provide reference data,

wherein the license plate tamper detection system is arranged to comparesaid monitor data with said reference data and generate a tamperingevent if said comparison yields more than a predetermined differencebetween the monitor data and the reference data.

Another aspect relates to a license plate tamper detection system forinstallation on a vehicle, the system comprising:

a first sensor for providing monitor data corresponding to presence of afirst license plate when installed on said vehicle; and

a second sensor for providing reference data when installed on saidvehicle,

wherein the license plate tamper detection system is arranged to comparesaid monitor data with said reference data and generate a tamperingevent if there is more than a predetermined difference between themonitor data and the reference data.

Thus it will be seen by those skilled in the art that theherein-described embodiments may provide a way of detecting attempts totamper with the license plate(s) on a vehicle by comparing data from onesensor associated with the license plate with reference data. If anattempt should be made to tamper with a license plate this is likely tobe experienced in a different manner by the first and second sensors,and so by comparing data from each of the devices it is possible todetermine if one has undergone a change that the other has not, whichmay be indicative of a tampering attempt. On the other hand more generalfactors such as temperature, noise, vibration etc. are likely to beexperienced similarly by both sensors and so do not trigger a tamperevent.

While in most countries worldwide a license plate is required on boththe front and rear of a vehicle, in others only one or the other isrequired. In fact in the United States, while the majority of statesrequire front and rear license plates, a significant number only requirea rear license plate. The first license plate—the presence of which isassociated with the monitor data provided by the first sensor—could betherefore be e.g. a front or rear plate.

Where the vehicle has only one license plate the second sensor may bemounted at any convenient location on the vehicle, away from the licenseplate such as within the boot (or trunk) or under the bonnet (or hood).

Where the vehicle has two license plates (or more) it could be decidedonly to monitor one but more desirably both are monitored. In a set ofembodiments a third sensor is arranged to provide second monitor datacorresponding to presence of a second license plate on the vehicle. Thesystem could be arranged to compare the second monitor data with thereference data and generate a tampering event if such comparison yieldsmore than a predetermined difference between the second monitor data andthe reference data. Alternatively a fourth sensor could be provided toprovide second reference data for this purpose.

In a preferred set of embodiments however the second sensor is arrangedto provide second monitor data corresponding to presence of a secondlicense plate on the vehicle. In other words the reference data isitself monitor data for a second license plate. This may provide asymmetrical arrangement in which monitor data from each sensor iscompared to the other.

While there are a number of possible locations where the sensor(s)providing monitor data corresponding to the presence of a license platecould be located, e.g. behind the respective license plate on thevehicle, in a set of embodiments at least the first sensor, andoptionally the second sensor, is embedded in or otherwise attached tothe corresponding license plate.

In a set of such embodiments, the tamper detection system is arranged toprovide information about the distance between the first and secondsensors. In such embodiments, the distance between the first and secondsensors should remain constant unless one of the sensors is moved, whichwould indicate that an attempt to tamper with the license plate in whichit is embedded or attached to has occurred.

There are a number of types of sensors that could be used to detect anattempt to tamper with the license plate(s). In a set of embodiments,the first and/or second sensors implement at least one sensor technologyfrom the following group: radar; ultrawideband radar; optical; magnetic;acoustic; pressure; accelerometers; gyroscopes.

The first and second sensors could communicate with an intermediate unitthat would perform the necessary comparisons and make decisions as towhether or not to generate a tamper event. In a set of embodimentshowever, the sensors are provided with local interface electronics toform sensor units which communicate directly with one another. Thisadvantageous arrangement allows the two devices to collaborate bysharing their respective data and making the relevant decisions withoutthe need for additional processing hardware. In a particular set ofembodiments, the communication between sensor units is wireless. Byusing wireless technology to implement sensor unit-to-sensor unitcommunications, the complexity of installation is reduced, makingretrofitting the system to older vehicles relatively straightforward. Ina set of embodiments, the communication implements at least one protocolfrom the following group: WiFi; ZigBee; Bluetooth®; Bluetooth® Smart(formerly known as Bluetooth® Low Energy); and ultra-wideband.

The Applicant has appreciated that a particularly convenient arrangementcan be achieved by providing sensors which are incorporated in licenseplates which can communicate with one another without requiring anyadditional hardware to be installed on the vehicle and so can be easilyretro-fitted to existing vehicles. In a second aspect, a license platetamper detection system is provided that has a first license platecomprising a first sensor arranged to provide first monitor data; and asecond license plate comprising a second sensor arranged to providesecond monitor data, wherein the first and second license plates arearranged to:

exchange said first and second monitor data;

compare said first and second monitor data; and

generate a tampering event if more than a predetermined differencebetween the first monitor data and the second monitor data.

It will be appreciated by those skilled in the art that when the licenseplates can thus form a standalone tamper detection system. Such licenseplates can be retrofitted to existing vehicles with relative ease, aswell as being suited to installation on new vehicles. For example, inembodiments powered by a battery, the license plates will not requireintegration into the electrical power system of the vehicle.

In either aspect, the first and second sensor units or the first andsecond license plates may be arranged such that one is a master whilethe other is a slave. This could mean that one is configured to carryout the processing to compare the first monitor data with the secondmonitor data. In another set of embodiments the first and second sensorunits or license plates are configured in the same way.

Particular embodiments include ultra-wideband radar for both sensingpresence of a license plate and communication between license plates orsensor units. Advantageously the sensor and communications shareultra-wideband hardware, thus reducing the bill of materials for eachsuch system.

A tampering event is generated if the (first) monitor data differs fromthe reference or second monitor data by a predetermined amount. Thepredetermined amount could be zero—i.e. as soon as the data from thefirst and second sensors do not correspond a tamper event may betriggered. However, in a set of embodiments it is non-zero. Thisadvantageous arrangement permits slight variations that are deemed to be“normal”, reducing the likelihood of the system reporting false positivetampering events.

The tampering events generated by the tamper detection system could bereported in real time. However, in a set of embodiments, the systemcomprises a memory arranged to store tampering events in an event log.In a further set of embodiments, the memory comprises non-volatilememory. In such embodiments, the system keeps track of any suchtampering events such that the log can be queried at a later time asrequired.

The log could be checked manually, for example by connecting an externaldevice such as a laptop computer to the system and downloading the eventlog. However, in a set of embodiments, the system further comprises acommunications system arranged to transmit data from said memory. Thecommunications system advantageously allows the tamper detection systemto communicate the log to external devices either automatically or uponrequest.

In a set of embodiments, the communications system is arranged tocommunicate with an enforcement device and provide tampering events fromthe event log to the enforcement device. In such embodiments, a lawenforcement agency such as the police may be provided with anenforcement device such as a handheld terminal that is arranged tocommunicate with the tamper detection system via its communicationsystem in order to establish whether any tampering events have occurred.

While it is useful for law enforcement to be able to interrogate theevent log manually using such a device as explained with reference tosome of the embodiments outlined above, in an overlapping set ofembodiments, the communications system may arranged to communicate withan automatic interrogation system and provide vehicle registrationinformation to the automatic interrogation system. In such embodiments,the communications system communicates with the automatic interrogationsystem which may comprise, by way of example only, an overhead gantrylocated above a motorway carriage or a roadside terminal located besidea tolling station. Such automatic interrogation systems may therefore bearranged to query the event log for each vehicle that passes it. Theseautomatic interrogation systems may be installed in strategic locationsas desired, or may include systems that are already in situ that arecapable of being retrofitted with the ability to interrogate the eventlog.

In a set of embodiments, the communications system is arranged toimplement dedicated short-range communications (DSRC).

The tamper detection system may also be arranged to provide additionalinformation via the communications system. By way of example only, thesystem may be arranged to communicate information relating to thelicense plate number, vehicle identification number (VIN), brand, model,colour, weight, size, etc.

Those skilled in the art will appreciate that a number of wirelessprotocols exist that would be appropriate for the communications systemoutlined in some embodiments described herein. In a set of embodiments,the communications system implements at least one protocol from thefollowing group: WiFi; ZigBee®; Bluetooth®; Bluetooth® Smart; andultra-wideband radar. Such protocols are widely adopted, thus makingintegration with existing hardware more feasible.

While the system could be controlled externally, in a set of embodimentsthe system comprises a microcontroller. Such embodiments allow thesystem to operate autonomously, performing all of the necessarycomparisons and deciding whether a tampering attempt has occurred. Themicrocontroller may also be arranged to store tamper events with theevent log and operate the communications system in embodiments includingsuch features.

The tamper detection system could be powered via the vehicle's own powersystems, such as via the 12 V or 24 V systems ubiquitous within theautomotive industry. However, this means that the system would bedraining the vehicle's battery (or batteries in the case of largertrucks) which are required for starting the vehicle. In an advantageousset of embodiments, the system comprises an internal battery. Theinternal battery ensures that the tamper detection system cannot bedisabled by simply cutting the external power, and allows the system tocontinue detecting attempts to tamper with the license plate(s) evenwhen the vehicle is not being operated. Other sources of power such asphoto-voltaic cells etc. are also possible.

While the various communication links (such as those between the firstand second sensor units, first and second license plates or between thetamper detection system and any remote enforcement system) may be open.In another set of embodiments however at least some of them areencrypted. Accordingly, the system may comprise an encryption module.The encryption module may be used to increase the security ofcommunications carried out by the tamper detection system usingcryptographic methods. Cryptography can be used to ensure thatinformation exchanged via the communication links is confidential (i.e.the information is accessible only to intended recipients), maintainsits integrity (i.e. the information is not altered in any manner),authenticated (i.e. the origin of the information is known withconfidence), and non-repudiable (i.e. the originator of the informationcannot deny that it was the originator). The encryption module mayemploy a number of different cryptographic functions known in the artper se, but may for example include block ciphers, stream ciphers, hashfunctions, digital signatures and random number generators.

Depending on the type of sensor in use, motion of the vehicle may causefalse positive tamper events. In a set of embodiments, the system isarranged to assess whether the vehicle is stationary. While thisassessment could be used to inform decision made about tampering, in aset of embodiments, the sensors are disabled if the vehicle is notstationary. Given that tampering with the license plate is highlyunlikely while the vehicle is in motion, disabling the system in suchcircumstances is of little concern to the security of the licenseplates.

In some embodiments, the first and second sensors compriseaccelerometers. The Applicant has appreciated that by comparing theacceleration experienced by the first license plate with that of thesecond license plate, any attempt to tamper with either of the platescan be successfully detected.

FIG. 1 is a block diagram of a tamper detection system 2 which includestwo sensor units 4, 6. Each of the sensor units 4, 6 comprises a sensor3 a, 3 b respectively. Each sensor unit 4, 6 also comprises amicrocontroller 8 a, 8 b; non-volatile memory 10 a, 10 b; acommunication system 12 a, 12 b; and are powered by a battery 16 a, 16b. The sensor units 3 a, 3 b are each connected to an enforcementcommunication system 14. While in this example each sensor unit 4, 6 hasits own battery 16 a, 16 b, other arrangements may utilise a singlebattery that powers the whole system 2.

While in this exemplary embodiment there is a single enforcementcommunication system 14 to which both sensor units 4, 6 are connected,other exemplary configurations have a separate enforcement communicationsystem for each sensor unit 4, 6 or a single enforcement communicationsystem within only one of the sensor units.

FIG. 2 shows schematically the approximate placement of two sensor units4, 6 on a vehicle 18. In this particular embodiment, the first sensorunit 4 is embedded in a license plate 20 affixed to the rear bumper 22of the vehicle 18 (as described further with reference to FIGS. 3A and3B) while the second sensor 6 is mounted under the bonnet 24.

In such an embodiment, the tamper detection system 2 is concerned onlywith attempts to remove the rear (and in this case only) license plate20 from the vehicle 18. As such, the second sensor unit 6 providesreference data—i.e. a baseline to which data from the first sensor unit4 can be compared.

FIGS. 3A and 3B show the sensor unit 4 embedded in the license plate 20.FIG. 3A depicts a front view of a license plate 20, and shows therelative placement of the sensor unit 4 (depicted as a dashed line) inthe license plate 20, although of course the sensor unit 4 could beplaced in any convenient part of the license plate 20. The license plate20 is securely attached to the rear bumper 22 of the vehicle 18 by fourscrews 26 a, 26 b, 26 c, 26 d. These screws 26 a, 26 b, 26 c, 26 d areoften arranged to make it difficult to remove the license plate 20,however this is not essential. The screws 26 a, 26 b, 26 c, 26 d maypenetrate the license plate 20 itself or may affix a frame (not shown)to the rear bumper 22 into which the license plate 20 is insertedaccording to national standards.

FIG. 3B shows a side view of the sensor unit 4 mounted embedded in thelicense plate 20.

The sensor in the sensor unit 4 is an accelerometer 28. Theaccelerometer is typically sensitive along three sensing axes 30,although this is not essential and fewer axes may be adequate. A localmicroprocessor (not shown) calculates the acceleration experienced bythe license plate 20 from the data provided by the accelerometer 28 in amanner known in the art per se. The sensor unit 4 also comprises one ormore radio antennas 36, the purpose of which will be described later.

The sensor unit 6 located under the bonnet 24 operates in a similar way.Thus an accelerometer associated with the sensor unit 6 located underthe bonnet 24 provides a reference measure of acceleration to which theacceleration data from the other sensor unit 4 may be compared.

The sensor units 4, 6 can share their respective acceleration data viathe communication system 12 (shown functionally in FIG. 1). Thecommunication system could use any appropriate wireless or wiredcommunication method known in the art but in this particular example thecommunication system 12 uses Bluetooth® to communicate data between thesensor units 4, 6. The data might be streamed in real-time between thesensors 4, 6 or it may be sent periodically, either as batches ofmeasured values or as a single time-averaged value.

The acceleration experienced by each sensor unit 4, 6 should normallyremain relatively stable. If there are fluctuations in the measurementsdue to vehicle-wide movements (for example wind buffeting or a personleaning against the vehicle) or due to changes in temperature, etc.,these will affect both sensor units 4, 6 equally and so a comparisonbetween them will yield little difference. The comparison could becarried out by either one of the sensor units 4, 6 or by both.

If an attempt is made to remove the license plate 20, by force or byremoving the screws 26 a-d, this is likely to cause the license plate 20to bend or move away from (or towards) the bumper 22 and theaccelerometer 28 within sensor unit 4 will sense the resultingacceleration. The acceleration data from the first sensor unit 4 willchange significantly compared to the acceleration data from the secondsensor unit 6 which would not change and a tamper event is thereforegenerated. Again either or both of the sensor units 4, 6 could generatethe tamper event and store it in an event log, kept on the non-volatilememory 10.

The content of the event log may be communicated to external devices bythe enforcement communications system 14 (shown functionally in FIG. 1).This comprises the antenna(s) 36 which is/are part of one of the sensorunits 4 and used by the enforcement communications system 14 to provideinformation to external devices (not shown) as will be described ingreater detail below with reference to FIG. 6 via dedicated short-rangecommunications (DSRC) or other communication protocol such as Bluetooth®or UWB.

FIG. 4 depicts, schematically, an alternative embodiment with adifferent physical arrangement but the same functional components asshown in FIG. 1. In this embodiment, the vehicle 118 is provided with arear license plate 120 a attached to a rear bumper 122 a, and a frontlicense plate 120 b attached to a front bumper 122 b. Two sensor units104, 106 are embedded within the respective license plates 102 a, 120 b.The system is therefore symmetric in that each sensor unit 104, 106 actsas reference for the other. The sensor units 104, 106 operate in thesame way as in the first embodiment and cooperate to determine whethertampering with either license plate 120 a, 120 b is taking place. Thetwo sensors 104, 106 share their respective acceleration data with oneanother via the communication system 12 as before. If there issignificant disagreement (i.e. more than a de minimus threshold) betweenthe acceleration data from the sensor units 104, 106 a tamper event isgenerated and stored in the event log as previously described.

FIG. 5 shows the tamper detection system 2 of FIG. 4 communicating witha handheld enforcement device 140, although the first embodiment maycommunicate in the same way. In this example, a police officer 142 hasdecided to perform a check on the vehicle 118 using a handheldenforcement device 140. The handheld enforcement device 140 uses DSRC150 (e.g. using an equivalent of the antenna 36 as illustrated in FIG.3B for transmission and reception) to interrogate the event log storedin the non-volatile memory 10 in order to determine whether any attemptsto tamper with the license plates 120 a, 120 b have been made. Thecommunications with the enforcement device 140 are encrypted to ensurethat security and confidentiality are maintained.

FIG. 6 shows the tamper detection system 2 of FIG. 4 communicating withan automatic interrogation system 144. In this example, the vehicle 118has driven past an overhead gantry 146 upon which the automaticinterrogation system 144 is mounted. In this exemplary embodiment, theautomatic interrogation device 140 again uses DSRC 150 to interrogatethe event log stored in the non-volatile memory 10. The automaticinterrogation device 144 may also include a camera 148, and opticalcharacter recognition (OCR) or other image processing techniques knownin the art per se may be utilised to check whether the data from theenforcement communications system 14 matches the information displayedon the license plates 120 a, 120 b. Again communications with theautomatic interrogation device 140 are encrypted.

FIGS. 7A and 7B show sensor units 204, 206 behind respective licenseplates 220 a, 220 b. FIG. 7A depicts a front view of a license plate 220a, 220 b, and shows the relative placement of the respective sensor unit204, 206 (depicted as a dashed line) behind the corresponding licenseplate 220 a, 220 b. The sensor unit 204, 206 may form an integral partof the license plate or license plate housing. The license plate 220 a,220 b is securely attached to a bumper 222 a, 222 b (front or rear) ofthe vehicle 218 by four screws 226 a, 226 b, 226 c, 226 d. These screws226 a, 226 b, 226 c, 226 d are often arranged to make it difficult toremove the license plates 220 a, 220 b. The screws 226 a, 226 b, 226 c,226 d may penetrate the license plate 220 itself or may affix a frame(not shown) to the bumpers 222 a, 222 b into which the license plates220 a, 220 b are inserted according to national standards.

FIG. 7B shows a side view of a sensor unit 204 mounted behind a licenseplate 220 a. The sensor unit 204 is mounted license plate 220 a via fourscrews 223 a, 223 b, 223 c, 223 d and faces the rear bumper 222 a. Theother sensor unit 206 may be mounted behind another license plate 220 bfacing a front bumper 222 b in a similar manner.

The sensor unit 204 comprises an ultra-wideband (UWB) radio transmitter228 and a UWB radio receiver 230. The UWB transmitter 228 transmitsshort, broad spectrum pulses 232 that are then reflected by the surfaceof the rear bumper 222 a. The reflected pulses 234 are then received bythe UWB receiver 230. A local microprocessor (not shown) calculates thetime of flight (TOF) by comparing the time between transmission andreception which then provides an indication of the distance to the rearbumper 222 a.

The sensor units 204, 206 can share the TOF data accumulated for theirrespective pulses via the communication system 212 (shown functionallyin FIG. 1). The communication system could actually use the UWBtransmitters 228 and receivers 230 with suitably encoded transmissions.Alternatively another wireless or wired channel could be used. The datamight be streamed in real-time between the sensors 204, 206 or it may besent periodically, either as batches of measured values or as a singletime-averaged value.

The TOF calculations by each sensor unit 204, 206 should normally remainrelatively stable. If there are fluctuations in the measurements due tochanges in temperature, etc., these will affect both sensor units 204,206 equally and so a comparison between them will yield littledifference. The comparison could be carried out by either one of thesensor units 204, 206 or by both.

If an attempt is made to remove a license plate 220 a, 220 b, by forceor by removing the screws 226 a-d, this is likely to cause the licenseplate 220 a, 220 b to bend or move away from (or towards) the respectivebumper 222 a, 222 b, increasing (or decreasing) the time betweentransmission and reception. The TOF data from the first sensor unit 204will change significantly compared to the TOF data from the secondsensor unit 206 which would not change and a tamper event is thereforegenerated. Again either or both of the sensor units 204, 206 couldgenerate the tamper event and store it in an event log, kept on thenon-volatile memory 210.

The UWB transmitters 228 and receivers 230 may also form part of theenforcement communications system as described previously, and thus canused for communicating the event log to external devices.

FIG. 8 shows time-of-flight based sensors embedded within two licenseplates as part of a tamper detection system 2. In this example twosensor units 304, 306 are embedded within rear and front license plates320 a, 320 b respectively. Each of the sensor units 304, 306 compriseUWB transmitters and receivers (not shown) as described with referenceto FIGS. 7A and 7B.

In this particular embodiment, the sensor units 304, 306 are eacharranged to calculate TOF data relating to the round trip of UWB signals300 therebetween. If either or both of the license plates 320 a, 320 bare moved, the distance between them will change, and thus a tamperingattempt may be detected. In some configurations only one of the sensorunits 304, 306 might calculate the TOF data, but by having both units304, 306 carry out the calculations, comparisons can be made by eitheror both of the units 304, 306 to ensure they are in agreement,increasing the reliability of the system.

Thus it will be seen that a license plate tamper detection system thatcompares data from a license plate sensor to data from a further sensorwhich may be located on an additional license plate or elsewhere on avehicle has been described herein. Although particular embodiments havebeen described in detail, it will be appreciated by those skilled in theart that many variations and modifications are possible using theprinciples set out herein. For example, tamper detection systemsincorporating other forms of sensors such as gyroscopes, magneticsensors, optical sensors, ultrasonic sensors, etc. are to be understoodas being within the scope of the instant disclosure.

Although the embodiments described have the sensor units communicatingdirectly with one another they could instead communicate with a furthercontrol unit which may be a dedicated control unit or part of thevehicle's management system.

What is claimed is:
 1. A license plate tamper detection system forinstallation on a vehicle, the system comprising: a first sensorarranged, when installed on the vehicle, to generate monitor datacorresponding to presence of a first license plate; and a second sensorarranged, when installed on the vehicle, to generate reference data,wherein the license plate tamper detection system is arranged to comparesaid monitor data with said reference data and generate a tamperingevent when there is more than a predetermined difference between themonitor data and the reference data; and wherein the first and secondsensors are arranged to communicate with each other using encryptedcommunications.
 2. The license plate tamper detection system as claimedin claim 1, wherein the first sensor is embedded in or otherwiseattached to the first license plate.
 3. The license plate tamperdetection system as claimed in claim 1, wherein the second sensor isarranged to provide second monitor data corresponding to presence of asecond license plate on the vehicle.
 4. The license plate tamperdetection system as claimed in claim 3, wherein the second sensor isembedded in or otherwise attached to the second license plate.
 5. Thelicense plate tamper detection system as claimed in claim 4, arranged toprovide information about a distance between the first and secondsensors.
 6. The license plate tamper detection system as claimed inclaim 1, wherein the first and second sensors comprise respective localinterface electronics to form sensor units which communicate directlywith one another.
 7. The license plate tamper detection system asclaimed in claim 1, adapted for wireless communication between the firstand second sensors.
 8. The license plate tamper detection system asclaimed in claim 7, arranged to use ultra-wideband radar for bothsensing presence of at least the first license plate and communicationbetween said first and second sensors.
 9. The license plate tamperdetection system as claimed in claim 1, comprising an encryption module.10. The license plate tamper detection system as claimed in claim 1,comprising a microcontroller.
 11. The license plate tamper detectionsystem as claimed in claim 1, comprising an internal battery.
 12. Thelicense plate tamper detection system as claimed in claim 1 arranged toassess whether the vehicle is stationary and to disable the sensors ifthe vehicle is not stationary.
 13. The license plate tamper detectionsystem as claimed in claim 1, wherein the first and second sensorscomprise accelerometers.
 14. The license plate tamper detection systemas claimed in claim 1, wherein the reference data does not correspond tothe presence of a license plate.
 15. A vehicle having a license platetamper detection system comprising: a first sensor, installed on thevehicle, arranged to generate monitor data corresponding to presence ofa first license plate on the vehicle; and a second sensor, installed onthe vehicle, arranged to generate reference data, wherein the licenseplate tamper detection system is arranged to compare said monitor datawith said reference data and generate a tampering event when saidcomparison yields more than a predetermined difference between themonitor data and the reference data; and wherein the first and secondsensors are arranged to communicate with each other using encryptedcommunications.
 16. The vehicle having the license plate tamperdetection system as claimed in claim 15, wherein the reference data doesnot correspond to the presence of a license plate.
 17. A license platetamper detection system for installation on a vehicle, the systemcomprising: a first sensor arranged, when installed on the vehicle, tosense monitor data corresponding to presence of a first license plate;and a second sensor arranged, when installed on the vehicle, to sensereference data, wherein the license plate tamper detection system isarranged to compare said monitor data with said reference data andgenerate a tampering event when there is more than a predetermineddifference between the monitor data and the reference data; and whereinthe first and second sensors are arranged to communicate with each otherusing encrypted communications.
 18. The license plate tamper detectionsystem as claimed in claim 17, wherein the reference data does notcorrespond to the presence of a license plate.